Method of alignment

ABSTRACT

The invention is concerned with a method f alignment for aligning crystal planes of a wafer substrate ( 40 ) to lithographic features thereon, the method characterised in that it includes the steps of: (a) measuring angular orientation of a peripheral flat ( 200 ) of the substrate ( 40 ); (b) measuring a crystallographic plane orientation of the substrate ( 40 ); (c) determining an error angle (φ) between the annular orientation of the flat ( 200 ) and the crystallographic orientation; (d) angularly registering to the flat ( 200 ) in a lithographic tool; (e) rotating the substrate ( 40 ) by the error angle (φ); and (f) defining one or more feature layers on the substrate ( 40 ) using the lithographic tool, thereby angularly aligning the one or more feature layers to the crystallographic plane orientation. The invention is further concerned with an apparatus for performing a method of alignment as claimed in claim  1,  the apparatus comprising: (a) a wafer flat measuring device ( 20 ) for measuring angular orientation of one or more peripheral flats ( 200 ) on a wafer substrate ( 40 ); (b) an X-ray diffractometer for measuring a crystallographic orientation of the substrate ( 40 ); and (c) a wafer chuck ( 100 ) for rotating the substrate relative to the wafer flat measuring device ( 1 ) and the X-ray diffractometer ( 30 ).

[0001] The present invention is concerned with a method of alignment, inparticular, but not exclusively, with a method of angularly aligningsemiconductor wafers during semiconductor lithographic processesassociated with fabricating semiconductor devices.

[0002] Semiconductor wafers are conventionally angularly aligned inlithographic tools, for example in step-and-repeat cameras, byregistering to one or more wafer flats formed in the wafers. Whenfabricating many types of semiconductor devices, for example logicdevices such as dynamic memories, alignment of integrated circuitfeatures, for example doped electrode regions and metal conductortracks, to wafer crystal planes is not especially critical to deviceperformance. However, high overlay accuracy of device layers isessential for achieving satisfactory device yield and operation.

[0003] In the case of micromachined devices, for example electro-opticaldevices such as solid-state lasers relying on the formation ofmirror-like cleaved surfaces as functional components thereof, highlyaccurate angular alignment of device features to wafer crystal planes iscritical. When this angular alignment is not accurately achieved, devicedimensional tolerances become difficult to attain and cleaved surfacescan often include stepped features which are deleterious to deviceperformance.

[0004] During conventional semiconductor wafer manufacture, grovesaligned to wafer crystal planes are scribed on wafers and the wafers arethen cleaved along the groves to form flats in the wafers. The wafersare then subjected to polishing operations because exposed abruptcleaved surfaces can render the wafers vulnerable to shatter duringsubsequent processing steps, for example high temperature dopantactivation processes, and can result in crystal dislocation defectspropagating through the wafers. The polishing processes are effective atsmoothing the abrupt cleaved surfaces.

[0005] The inventor has appreciated that manufacturing yield ofmicromachined electro-optical devices manufactured by registering topolished wafer flats is adversely affected by the alignment accuracy ofthe polished flats to associated wafer crystal planes. The inventor hasfurther appreciated that, in practice, registration to the polishedwafer flats can be repeatedly achieved to an accuracy comparable to thatrequired to the wafer crystal planes. Thus, the inventor has therefromdevised an alignment method according to the invention.

[0006] Thus, according to a first aspect of the present invention, thereis provided a method of alignment for aligning crystal planes of a wafersubstrate to lithographic features thereon, the method characterised inthat it includes the steps of:

[0007] (a) measuring angular orientation of a peripheral flat of thesubstrate;

[0008] (b) measuring a crystallographic plane orientation of thesubstrate;

[0009] (c) determining an error angle between the angular orientation ofthe flat and the crystallographic orientation;

[0010] (d) angularly registering to the flat in a lithographic tool;

[0011] (e) rotating the substrate by the error angle; and

[0012] (f) defining one or more feature layers on the substrate usingthe lithographic tool, thereby angularly aligning the one or morefeature layers to the crystallographic plane orientation.

[0013] The method provides the advantage that it is capable of moreaccurately aligning the feature layers to the crystallographic planeorientation.

[0014] Preferably, the crystallographic plane orientation is measuredusing X-ray diffraction means. X-ray diffraction techniques provide thebenefit of non-contact non-invasive measurement of crystal planeorientation. Moreover, such techniques can be performed rapidly which isimportant in semiconductor manufacturing environments. It is especiallypreferable that the X-ray diffraction means is operable to interrogate aperipheral edge of the substrate.

[0015] The inventor has found it preferable to perform theaforementioned method of the invention by employing an X-ray diffractionmeans comprising an X-ray source for generating X-ray radiation, a beamconditioner for forming the X-ray radiation into an X-ray beam directedtowards the substrate, and an X-ray detector for receiving a diffractedportion of the beam emitted from the substrate. Moreover, the inventorhas found it preferable to arrange the diffraction means so that thecrystallographic orientation of the substrate is determined bymaximising an X-ray radiation count in the detector as the substrate isrotated.

[0016] In a second aspect of the present invention, there is provided anapparatus for performing a method of alignment according to the firstaspect, the apparatus comprising:

[0017] (a) first measuring means for measuring angular orientation ofone or more peripheral flats on a wafer substrate;

[0018] (b) second measuring means for measuring a crystallographicorientation of the substrate; and

[0019] (c) substrate rotating means for rotating the substrate relativeto the first and second measuring means.

[0020] Preferably, the second measuring means is an X-ray diffractometerdevice. Such a device is capable of providing non-contact non-invasiveand rapid measurement of crystallographic orientation.

[0021] Embodiments of the invention will now be described, by way ofexample only, with reference to the following diagrams in which:

[0022]FIG. 1 is a schematic illustration of an alignment apparatus foruse in the method according to the invention.

[0023] It is well known that semiconductor wafers are cut fromsingle-crystal ingots grown in crystal growing apparatus. The ingots canbe grown in a number of alternative crystal orientations. Theretherefore arises a need within the semiconductor industry to have amanner of marking wafers to indicate their respective crystalorientation. Such marling is conventionally achieved by forming flats atperipheral edges of wafers. The flats are employed for coarselyorientating the wafers in automatic processing equipment duringsemiconductor device manufacture prior to applying precisionregistration processes based on optical inspection of surface featureson one or major polished faces of the wafer to ensure precise devicelayer overlay registration. A problem arises for an initial featurelayer to be formed on one or more of the major faces where noregistration features preexist other than the aforementioned flats.

[0024] When semiconductor devices such as memory devices are fabricated,angular alignment of device features to wafer crystal planes are notcritical; however, high speed logic devices fabricated in certaincrystal orientations tend to function more rapidly because electron andhole mobilities are slightly anisotropic in silicon and III-V compounds.When such memory devices are fabricated, polished wafer flats provide asufficiently accurate angular alignment when forming initial layers.

[0025] A particular problem of angular alignment of initial featurelayers arises in the case of micromachined semiconductor devices whichrely on device features being accurately aligned to wafer crystalplanes. The problem is pertinent in the case of silicon wafers which areto be anisotropically etched using, for example, potassium hydroxidesolution to form device features; moreover, the problem is also relevantin the case of III-V compound wafers which are to be cleaved to yieldmirror-like planes for electro-optical devices.

[0026] There are a plurality of established conventions concerning theposition and size of wafer flats. For 50 mm diameter wafers followingthe conventions, each wafer includes a major flat of length 16±2 mmroughly parallel to the <110> crystal plane, and a minor flat of length8±1 mm roughly parallel to the <110> crystal plane. As described in theforegoing, these flats are found in practice by the inventor to beinsufficiently accurately aligned to the crystal planes to be reliedupon for angular registration when delineating initial device layers ofelectro-optical devices incorporating cleaved mirrorlike planes.However, the inventor has unexpected found that registration to polishedwafer flats can be achieved to a high degree of accuracy andrepeatability using contemporary wafer-flat optical or mechanicalregistration tools.

[0027] Thus, the inventor has appreciated that prior to delineating aninitial device layer on a semiconductor wafer, it is feasible to measurean error angle between one or more wafer flats of the wafer and itscrystal orientation. The error angle can be used as a data parameterwhich accompanies the wafer in its subsequent fabrication processingsteps; the error angle can be used as a correction angle to be appliedafter angularly registering to the one or more flats to ensure thatinitial delineated features formed on the wafer are more accuratelyangularly aligned to the crystal planes of the wafer than possible ifonly registration to the flats were relied upon.

[0028] The method of the invention will now be described in more detailwith reference to FIG. 1. In FIG. 1, there is shown an alignmentapparatus for use in the method, the apparatus indicated generally by10. The apparatus 10 comprises a conventional optical flat registrationdevice 20 and an X-ray diffractometer indicated by 30. The registrationdevice 20 is, for example, a proprietary device type ? manufactured by?. The diffractometer 30 is positioned relative to a wafer 40 mounted ona rotatable wafer chuck 50 for measuring crystal plane orientation ofthe wafer 40 relative to the diffractometer 30.

[0029] The diffractometer 30 includes an arcuate track 100 onto which ismounted a first assembly and a second assembly. The first assemblycomprises an X-ray radiation source 120 coupled to an associated X-raybeam conditioner 130. The second assembly includes an X-ray detector150. The assemblies are slidably adjustable along the track 100 forvarying an angle θ at which an X-ray beam 140 emitted from the firstassembly and reflected from a peripheral edge of the wafer 40 subtendswith respect to a central axis 160 as shown in FIG. 1. The axis 160 isradially orientated with respect to a centre C of the wafer 40 and itsassociated chuck 50. In operation, the angle θ is varied depending uponthe order of X-ray diffraction employed, the free-space wavelength ofX-ray radiation forming the beam 140 and the material from which thewafer 40 is formed.

[0030] Operation of the apparatus 10 for determining an angular errorbetween a wafer flat 200 on the wafer and wafer crystal orientation willnow be described with reference to FIG. 1.

Step 1

[0031] Initial calibration of the apparatus 10 is required. Suchcalibration is achieved using a calibration wafer. The calibration waferis prepared by taking a semiconductor wafer including a minor flat, forexample a flat of 8±1 mm length in a 50 mm diameter wafer, scribing agrove parallel to the flat further towards the centre C of the wafer andthen cleaving the calibration wafer at the grove to present amirror-quality <110> abrupt cleaved edge 200 to the wafer which isaccurately aligned with respect to crystal planes within the wafer. Thecleaved edge preferably has a length of 16±2 mm, namely similar in sizeto normal major wafer flat

[0032] The calibration wafer is mounted on the chuck 100. The cleavededge 200 is then offered to the flat registration device 20 and thechuck 100 rotated until the cleaved edge is angularly aligned to thedevice 20, namely the edge 200 is parallel to an axis X-Y. Thediffractometer 30 is then rotated as a complete assembly about thecentre C until an X-ray count rate of the detector 150 is maximalcorresponding to alignment of the diffractometer 30 to <110> crystalplanes of the calibration wafer.

[0033] This STEP 1 ensures that that the flat registration device 20 andthe diffractometer 30 are mutually angularly aligned, since both arealigned to the <110> crystal plane of the calibration wafer.

Step 2

[0034] The production wafer 40 is next mounted onto the chuck 100 insubstitution of the calibration wafer, the wafer 40 having a polishedflat thereon. The registration device 20 measures the alignment of thepolished flat 200 of the wafer 40 and the chuck 100 is rotated until thepolished flat is aligned to the X-Y axis of the device 20. The chuck 100is then rotated by an angle φ whereat the X-ray count rate of the X-raydetector 150 is maximal. The angle φ is then an error angle between the<110> crystal planes of the production wafer 40 and its associated flat200.

[0035] This angle φ for the production wafer 40 is a parameter whichaccompanies the wafer 40 through its subsequent production processes.

[0036] STEPS 1 and 2 above are then complete.

[0037] In electro-optical device fabrication, it is a first lithographicstep which is critical to ensuring that delineated features are alignedto crystal planes of a wafer. Thus, when projecting a first-layer maskimage in a lithographic tool, for example using a step-and-repeatcamera, to delineate a first layer of features onto the production wafer40, the wafer 40 is placed on a wafer chuck of the tool and a wafer flatregistration device of the tool is invoked to align the polished flat200 of the wafer 40 to the device. The wafer chuck of the tool is thenrotated by the angle φ so that crystal planes of the wafer 40 arealigned with respect to features in a lithographic mask loaded into thetool for delineating the first layer of features onto the wafer 40.Thus, by rotating the wafer 40 by the angle φ in the lithographic tool,improved feature-to-crystal plane alignment is achieved; such improvedalignment is important for achieving improved electro-optical -deviceyields from the wafer 40. If such an angular correction were notapplied, device yield from the wafer 40 would be lower.

[0038] Subsequent feature layers projected and formed onto the wafer areregistered with respect to the first feature layer and are therebyangularly aligned to the crystal planes of the wafer 40.

[0039] It will be appreciated that modifications can be made to theapparatus 10 and also to the aforementioned method of the inventionwithout departing from the scope of the invention. For example, theconfiguration of the diffractometer 30 can be changed provided that itis capable of measuring wafer crystal plane orientation.

1. A method of alignment for aligning crystal planes of a wafersubstrate to lithographic features thereon, the method characterised inthat it includes the steps of: (a) measuring angular orientation of aperipheral flat of the substrate; (b) measuring a crystallographic planeorientation of the substrate; (c) determining an error angle between theangular orientation of the flat and the crystallographic orientation;(d) angularly registering to the flat in a lithographic tool; (e)rotating the substrate by the error angle; and (f) defining one or morefeature layers on the substrate using the lithographic tool, therebyangularly aligning the one or more feature layers to thecrystallographic plane orientation.
 2. A method according to claim 1wherein the crystallographic plane orientation is measured using X-raydiffraction means.
 3. A method according to claim 1 or 2 wherein theX-ray diffraction means is operable to interrogate a peripheral edge ofthe substrate.
 4. A method according to claim 2 or 3 wherein the X-raydiffraction means comprises an X-ray source for generating X-rayradiation, a beam conditioner for forming the X-ray radiation into anX-ray beam directed towards the substrate, and an X-ray detector forreceiving a diffracted portion of the beam emitted from the substrate.5. A method according to claim 4 wherein the crystallographicorientation of the substrate is determined by maximising an X-rayradiation count in the detector as the substrate is rotated.
 6. Anapparatus for performing a method of alignment as claimed in claim 1,the apparatus comprising: (a) first measuring means for measuringangular orientation of one or more peripheral flats on a wafersubstrate; (b) second measuring means for measuring a crystallographicorientation of the substrate; and (c) substrate rotating means forrotating the substrate relative to the first and second measuring means.7. An apparatus according to claim 6 wherein the second measuring meansis an X-ray diffractometer device.
 8. A method of alignmentsubstantially as hereinbefore described with reference to FIG.
 1. 9. Anapparatus for aligning a wafer substrate, the apparatus substantially ashereinbefore described with reference to FIG. 1.